The invention relates to a color display device comprising an in-line electron gun for generating three electron beams, and a unit for deflecting the electron beams across a color selection electrode.
Such display devices are known.
A current aim is to make the outer surface of the color display window flatter, so that the image displayed on the display window is perceived by the viewer as flat. However, an increase of a radius of curvature of the outer surface will lead to an increase of a number of problems. The radius of curvature of the inner surface of the display window and of the color selection electrode should increase, and, as the color selection electrode becomes flatter, the mechanical strength of the color selection electrode decreases and hence the sensitivity to doming and vibrations increases. An alternative solution to this problem would be to curve the inner surface of the display window more strongly than the outer surface. By virtue thereof, a color selection electrode having a relatively small radius of curvature can be used. As a result, doming and vibration problems are reduced, but other problems occur instead. The display window has a much larger thickness at the edges than in the center. As a result, the weight of the display window increases and the intensity of the image decreases substantially towards the edges.
EP 0,421,523 discloses a color cathode ray tube with an in-line gun, a pincushion correcting yoke and an eyebrow effect electron-optical distortion correction device comprising two pairs of coils, each pair having a coil on each outer electron beam side of the neck in the plane of the beams. The coil pairs are spaced apart along the z-axis between the gun and the yoke and are driven by a sawtooth current having a bow-tie envelope synchronous with the raster scan to correct the dynamic, asymmetrical eyebrow effect apparent as a purity defect on the raster.
WO 99/34392 describes a color display device comprising a color cathode ray tube including an in-line electron gun for generating three electron beams, a color selection electrode, deflecting means for deflecting the electron beams located at a deflection plane, and first and second influencing means to dynamically influence the convergence of the electron beams, to decrease a distance between the electron beams at a location of the deflection plane.
The presence of the influencing means may introduce some undesired image artifacts. The so-called East/West raster distortion may be influenced, as well as the linearity of the deflection of the electron beams in a vertical direction. In conventional cathode ray tubes, deflection in the vertical direction is not a linear function of the deflection current. This non-linearity is corrected electronically. Introduction of the first and second influencing means leads to a cathode ray tube in which the non-linearity is reduced or is even overcompensated. These effects are undesired.
It is an object of the invention to provide an improved color display device. To this end, the invention provides a color display device and a deflection unit as defined in the independent claims. The dependent claims describe advantageous embodiments.